Understanding Strobes Takes Your Underwater Shots to the Next Level
Unerwater strobes are one of the most important parts of an effective underwater photography rig. The light they emit is what gives an underwater image its color, and without a strobe, resulting images typically look mostly blue or green. Also, external strobes are positioned away from the camera itself, which helps to reduce "backscatter," the unintentional lighting of small, floating particles in the water.
Once you have decided that an underwater strobe is necessary, the fun begins. Strobes are named with a combination of acronyms and numbers, and their feature lists include words like "Auto" and "Slave," and no less than five flavors of TTL (automatic "through the lens" metering). We've even talked with professional photographers who are confused about the current lineup of strobes. So how does one go about actually picking one?
We like to divide the strobe world into three categories: point-and-shoot strobes, manual strobes and modern strobes. The small, low-powered strobes are often bundled with point-and-shoot housings because of their limited usefulness. These are mostly effective for shooting macro with point-and-shoot cameras.
"Manual" strobes cover a larger range, mostly models that film shooters used years ago. Per their categorization, these strobes are used completely in manual mode, and work with any camera that uses a hot shoe to trigger a flash. However, not all of these strobes work with all cameras, and you should confirm that your specific camera model and housing are compatible with the strobe before purchasing one.
Finally, "modern" strobes are current strobe models that support all of the fancy electronics and flash patterns that digital cameras employ. The most recent strobes support both iTTL and E-TTL, Nikon and Canon's automatic flash exposure protocols. Some strobes implement their own, emulated TTL protocols, and some use the "Auto" nomenclature, which is different altogether. Most modern strobes also allow for manual override, which is important for situations where an automatic flash exposure isn't appropriate.
CHOOSING A STROBE Different photographers have different strobe needs.
Most point-and-shoot camera/housing setups allow the internal flash to be used; for macro photography in clear water, an on-board flash may be perfectly adequate. However, as soon as one starts shooting wide-angle, an external strobe becomes necessary. Most point-and-shoot cameras only work with strobes that feature optical slave sync. A fiber-optic sync kit will ensure that your strobes don't fire when other photographers are shooting.
Many photographers shoot with two strobes on articulating arms. SLRs need more light than do point-and-shoots; divers who shoot predominantly macro can get away with using strobes that are powerful and narrow, but wide-angle shooters will need ones that are both powerful and wide.
The Ikelite DS51 is a macro strobe with a small, powerful, narrow beam and uses AA batteries (ikelite.com); the Ikelite DS-125 is flexible and powerful for macro and wideangle, and uses rechargeable battery packs; the Sea&Sea YS-90 TTL Duo is small, uses AA batteries and supports optical sync (seaandsea.com); the INON D-2000/Z- 240, powerful for its size, has a wide beam, uses AA batteries and supports optical sync (inonamerica.com); and the universal SeaLife Digital Pro Flash SL961 has an auto mode, optimum distance of two to eight feet and uses AA batteries (sealife-camer as.com).
Visit sportdiver.com/strobes, for more information.
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GLOSSARYAuto Automatic flash exposure measured by setting an f-stop on the strobe itself, which measures reflected light and determines how much light to produce based on the f-stop setting.
Slave (optical) The strobe fires when it sees a flash, and varies its power by reading the power of the flash it sees. The problem with slave strobes is that they fire when other people's strobes go off as well!
Guide Number The rated power of a strobe at full dump. The formula to remember is: Guide number = distance X f-stop. Guide numbers are specified in feet or meters, topside or underwater. For example, a GN of 32 (meters, topside) means that at full power, the strobe will provide enough light to expose at f32, one meter away, or at f8, four meters away, on land. Guide numbers are usually specified at a film sensitivity of ISO 100.
A common mistake photographers make is that they point their strobes directly at the subject. To reduce backscatter, move your strobes away from the camera housing and angle them outward so that you are illuminating your subject with the edge of the strobe beam. Your goal is to not light up any of the water between you and your subject.
With MacroAutomatic strobe exposure with TTL is very effective in macro photography. Pull your strobes in tight to maximize power and shoot away! If you prefer to shoot using manual strobe exposure, start with a preset exposure on your camera (try 1/125 sec @ f16) and set your strobes to maximum power. Look at your resulting image and adjust the aperture and strobe power accordingly.
Shooting wideangle can be tricky. Follow the steps outlined at left to reduce backscatter, and experiment with diffusers and manu al power settings. Your goal is to both illuminate evenly and light up the scene in a way that allows shadows to create interesting textures. It's easier said than done, but with enough practice, you'll start to get it!
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